Separating acetylene from gas mixtures



June 20, 1967 H. FRlZ ETAL 3,325,972

SEPARATING ACETYLENE FROM GAS MIXTURES Filed April 17, 1964 s Sheets-Sheet 1 FIG. I

L i? 7- 8 l3 2| mvzmons: HANS FRIZ MARTIN REICHERT ULRICH WAGNER ATT'YS m 1967 H. FRIZ ETAL SEPARATING ACETYLENE FROM GAS MIXTURES Filed April 17. 1964 5 Sheets-Sheet 2 s R O T l N w 7 a z mm 0 2 c 8 L 2 l B 3 2 2\ l RR M EE HN CG 1 MB RW F N H MUM R AAL HMU B ATT'YS June 20, 1967 z ETAL 3,325,972

SEPARATING ACETYLENE FROM GAS MIXTURES Filed April 17, 1964 3 Sheets-Sheet 3 I INVENTORS: HANS FRQ MARHN RDCHERT ULRmH WMGNER ATT'YS United States Patent This invention relates to a process of purifying gases containing acetylene. More particularly, the invention relates to a process of recovering pure acetylene from gas mixtures containing acetylene.

Methods are known for the separation of acetylene from gas mixtures, particularly from gas mixtures which have been obtained by thermal or electric cracking of hydrocarbons with or without oxygen, in which the acetylene is washed out from the gas mixture by a solvent. The

washing is carried out at superatmospheric pressure so that a maximum quantity of acetylene is dissolved. The dissolved acetylene is liberated from the wash liquor by decreasing the pressure or by raising the temperature or by both measures together. It can then be freed from compounds having lower solubility than acetylene in countercurrent to the washing liquid.

When acetylene is washed out in this way, not only acetylene but also compounds having better solubility than acetylene (for example methylacetylene, vinylacetylene and diacetylene) are separated. These may be expelled from the wash liquor after the acetylene has been gassed out, at elevated temperature and preferably at subatmospheric pressure by means of steam and withdrawn as a mixture with steam. The degassed wash liquor is then used again for washing out acetylene (Chem. Ing. Technik, 26, 253 1954)).

It is furthermore known that the gas obtained by cracking hydrocarbons may be washed, prior to the compression and the washing out of the acetylene, with a small amount of the same solvent (preliminary washing) so that compounds are removed from the gas which would cause soiling and stoppages in the compressor. These are particularly polycyclic hydrocarbons, for example naphthalene, methylnaphthalenes, acenaphthalene and acenaphtheme. The small amounts of wash liquor laden with these compounds obtained by the preliminary washing are freed from the absorbed substances together with the larger amounts of wash liquor used for washing out the acetylene under pressure (main washing). Combination of the two wash liquors in the said method advantageously takes place after the acetylene liberated from the larger amount of wash liquor has been purified countercurrent to the wash liquor from the main washing.

Another proposal has been first to wash out from the acetylene-containing gas mixture at atmospheric pressure or at superatrnospheric pressure, the whole of the diacetylene with a small amount of solvent, to free the wash liquor laden with diacetylene from the diacetylene by stripping with a gas or steam, to wash out the acetylene from the gas mixture free from diacetylene with a larger amount of the same solvent under the same pressure, and to expel the dissolved acetylene again by heating the wash liquor.

Suitable solvents for this process are for example butyrolactone and N-methylpyrrolidone.

In the separation of acetylene from gas mixtures by the prior art methods, polymerisation products form in the wash liquor and cannot be removed from the wash liquor by degassing. A portion of these polymers remain dissolved in the Wash liquor and must be removed therefrom by complete or partial distillation of the wash liquor. Another portion of the polymers is deposited in solid form, for example in heat exchangers and columns, so that heat transfer is impaired and passage of gas and liquids made more difiicult.

It is an object of the present invention to provide a process for the purification of gases containing acetylene, in which distillation of the wash liquor is necessary only to a small extent. Another object is to ensure that only small amounts of deposits occur in the Wash liquor, heavy contamination of the wash liquor by solids thus being avoided. A further object of the invention is to provide a process by which only small amounts of non-volatile substances are formed in the wash liquor and the wash liquor can be easily freed from substances dissolved therein, for example by heating. Other objects and advantages of the invention will be apparent from the following detailed description of our invention.

The said objects are achieved by bringing the wash liquor obtained from the preliminary washing into contact, preferably countercurrent, with the mixture of steam and/ or solvent vapor and compounds having better solubility than acetylene which is obtained by expelling the compounds having better solubility than acetylene from the solvent of the main washing. The wash liquor from the preliminary washing which has been treated in this way is advantageously used again as wash liquor either alone or together with the wash liquor from the main washing which has been freed from acetylene and compounds having better solubility than acetylene.

It is advantageous to remove constituents of low volatility by distillation from the wash liquor or from a portion thereof and to use again for the two washings the purified wash liquor together with the degassed wash liquor from the main washing and optionally the remainder of the wash liquor from the preliminary washing.

It is advantageous partly or completely to remove any water contained in the solvent to he returned to the preliminary washing.

It is sufiicient for the amount of solvent for the preliminary washing to be such that only a portion of the diacetylene is absorbed in the preliminary washing. The same effect is however achieved if the whole of the diacetylene is washed out from the cracked gas in the preliminary washing. Any small amount of acetylene absorbed during the preliminary washing may be removed again from the solvent in a conventional way by stripping the solvent from the preliminary washing with an inert gas (for example a portion of the lean gas practically free from acetylene obtained in the main washing) and returned in front of the compressor.

The following examples, given with reference to the accompanying drawings, will further illustrate the invention.

3 Acetylene 8.4 C H ;hydrocarbons 0.13 Vinylacetylene 0.04 Diacetylene 0.23 Ethylene 0.2 Methane 3.9 Carbon dioxide 3.8 Carbon monoxide 37.8 Hydrogen 43.5 Nitrogen 1.9 Oxygen 0.1

Naphthalene, acenaphthene and acenaphthylene are also detected in small concentration in the gas mixture.

640 cubic metres (S.T.P.) per hour of this gas is passed at a pressure of 1.03 atm. abs. and at a temperature of 25 C. through line 1 into a pre-scrubber 2 in an apparatus as shown diagrammatically in FIGURE 1. 125 kg. per hour of N-methylpyrrolidone is supplied through line 3. A gas containing 0.15% by volume of diacetylene is drawn ofl through line 4. 34.8% of the diacetylene has thus been washed out. The washed gas is compressed in a compressor 5 .to atm. abs. and washed in absorber 6 with 3900 kg. per hour of N-methylpyrrolidone which flows in at 28 C. through line 7. 583 cubic metres (S.T.P.) per hour of gas having the following average composition in percent by volume:

Acetylene 0. 1 Ethylene 0.2 Methane 4.3 Carbon dioxide 4.2 Carbon monoxide 41.4 Hydrogen 47.6 Oxygen 0.1 Nitrogen 2.1

is drawn otfthrough line'8.

Washing agent laden with acetylene and a portion of the other constituents contained in the crude gas leaves tower 6 through line 9, passes while being flashed to 1.2 atm. abs. into countercurrent stipper 10 and thence through line 11 into degassing tower 12 where the dissolved gases are liberated by heating to 100 C. and decreasing the pressure to 0.2 atm. abs. They pass through line 13 into the lower portion of the countercurrent stripper 10 in which they flow countercurrent to the descending washing agent. Some of the acetylene leaves tower 10 with the gases expelled from the solution through line 14 and is supplied to the suction line of compressor 5. 53 cubic metres (S.T.P.) per hour of acetylene containing less than 0.1% of carbon dioxide is withdrawn through 'line 15.

The constituents of the gas which are more soluble than acetelene (besides small amounts of acetylene, the constituents methacetylene, Vinylacetylene and diacetylene are detected in this gas) escape from degassing tower 12 through line 16 and enter at the bottom of column 17 as a mixture with water vapor and N-methylpyrrolidone vapor. The laden Wash liquor from the pro-scrubber is in-' troduccd into the top of column 17 through line 29. The mixture of vapor and gas Withdrawn from the top of the column 17 through line 18 is passed into column 19 to which 100 kg. per hour of water is supplied through line 20 and washes out the N-methylpyrrolidone vapor and the mixture of water vapor and gas is withdrawn through line 21. The mixture of N-methylpyrrolidone and water flowing away through line 22 is passed to the top of degassing tower 12 in the bottom of which the vapor leaving through line 16 is produced by heating. The wash liquor degassed in column 12 is passed through line 23 and 7 mainly into the top of absorber 6, the remainder flowing through line 3 to the top of the prescrubber 2. Water contained in the wash liquor flowing down in column 17 is expelled in the 'bottom of the column by heating and the wash liquor leaves the column through line 24 from which a bleed stream of 60 kg. per hour is withdrawn for distillation in a still 25, the remainder flowing through line 26 to the top of prc-scrubber 2. The distillate from still 25, after condensation and cooling, flows through lines 27 and 7 to the top of absorber 6.012 kg. per hour of non-volatile constituents is withdrawn at 28.

The wash liq-nor contains 0.06% by weight of dissolved polymers at the bottom of degassing tower 12 and 0.2% by weight of dissolved polymers at the bottom of column 17.

For comparison, theprior art method and the results achieved with it are hereinafter described:

A gas mixture having the, following average composi tion in percent by volume is producedby partial oxidation of hydrocarbons:

Naphthalene, acenaphthylene and acenaphthene are al- 'so detected in small concentration in the gas mixture.

640 cubic meters (S.T.P.) per hour of this gas is passed into an apparatus as shown diagrammatically in FIG- URE 2 (prior art method) at a pressure of 1.03 atm. abs. and at a temperature of 25 C. through line 1 into pre-scrubber 2.165 kg. per hour of. N methylpyrrolidone is supplied through line 3. A gas containing 0.13% by volume of diacetylene is withdrawn through line 4, so that 43.5% of the diacetylene has been washed out in column 2."This gas is compressed to 9 atm. abs. in compressor 5 and washed in absorber 6 with 4330 kg. per hour of N-methylpyrrolidone which flows through line 7 at 27 C. 580 cubic meters (S.T.P.) per hour of a gas having the following average composition in percent by volume is withdrawn through line 8:-

Acetylene 0.1 Ethylene 0.2 Methane 4.3 Carbon dioxide. 3.9 Carbon monoxide 41.8 Hydrogen 48.1 Oxygen 0.1 Nitrogen 1.5

The laden wash liquor from the pre-scrubber leaves column 2 through line 29 and is mixed in line 11 with the solution from countercurrent stripper 10. The bulk of the wash liquor degassed in degassing tower 12 is resupplied through line 23 and 7 or v3 to the two washing stages, while a bleed stream of 60 kg. per hour is withdrawn for distillation in still 25. The distillate from still 25 is condensed and cooled and passed through line 27 and united with the wash liquor flowing in line 23. 0.52 kg. per hour of non-volatile polymers is withdrawn at 28. The wash liquor at the bottom of degassing tower 12 contains 0.87% by weight of dissolved polymers. kg. per hour of water, is supplied at 20.

57.3 cubic meters (S.T.P.) per hour of acetylene containing less than 0.1% of carbon dioxide is withdrawn through line 15.

The remainder of the apparatus has the same function as in FIGURE 1.

By comparing the amount of non-volatile constituents withdrawn at point 28 (0.12 kg. per hour) and the amount of polymers contained in the bottom of degassing tower 12 (0.06% by weight) in the process according to this invention with the corresponding values (0.52 kg. per hour and 0.87% by weight) obtained by the prior art method the technical advance associated with the process according to this invention may be clearly seen.

EXAMPLE 2 640 cubic meters (S.T.P.) per hour of the same acetylene-containing cracked gas as is described in Example 1 is washed with 200 kg. per hour of N-methylpyrrolidone in column 2 of the apparatus illustrated in FIGURE 3 under otherwise the same conditions.

The gas withdrawn through line 4 contains 0.0069% by volume of diacetylene so that 97% of the diacetylene has been washed out.

The laden wash liquor leaving pre-scrubber 2 through line 29 is freed from dissolved acetylene in column 30 by means of 8 cubic meters (S.T.P.) per hour of the gas escaping from absorber 6 after the absorption of acetylene, which is introduced through line 31. The gas containing acetylene leaving the top of column 30 is recycled through line 32 to the suction side of compressor 5. The solvent flowing away from column 30 is passed through line 33 to the top of column 17. 0.162 kg. per hour of dissolved polymers is withdrawn at 28.

The wash liquor contains 0.073% by weight of nonvolatile polymers at the bottom of the degassing tower 12, and 0.27% by weight of non-volatile polymers at the bottom of column 17. The amounts, pressures and temperatures and also the function of the apparatus in FIG- URE 3 correspond to those given in Example 1 and FIG- URE 1. Again, the low content of polymers in the wash liquor when working according to the invention is evident.

EXAMPLE 3 640 cubic meters (S.T.P.) per hour of the same cracked gas as is described in Example 1 but in which no polycyclic aromatics are contained is compressed to atm. abs. in compressor 5 of an apparatus as shown diagrammatically in FIGURE 4 and passed through line 34 into scrubber 35. 116 kg. per hour of N-methylpyrrolidone is added at a temperaature of 30 C. to the top of the scrubber 35. Diacetylene cannot be detected in the pre-washed gas flow through line 37 into absorber 6.

The laden wash liquor from pre-scru'bber 35 flows through line 38 into column 30 while being flashed to 1.2 atm. abs. 4.3 cubic meters (S.T.P.) of the gas leaving absorber 6 is introduced per hour into column 30.

The wash liquor leaving column 30 is passed through line 33 to the top of column 17 and the discharge from column 17 passes through line 40 into degassing tower 12. The degassed wash liquor from the bottom of column 12 is for the most part recycled through lines 23 and 7 or 36 to the two scrubbers, while a bleed stream of 60 kg. per hour is withdrawn for distillation in still 25. After condensation and cooling, the distillate is withdrawn through line 27 and united with the wash liquor flowing in line 23.

47 kg. per hour of water is supplied at to the top of column 19.

The wash liquor in the bottom of column 12 contains 0.12% by weight of dissolved polymers and 0.126 kg. per hour of non-volatile polymers is withdrawn at 28.

The other amounts, pressures and temperatures and also the function of the apparatus correspond to those in Example 1 and FIGURE 1.

For comparison, a prior art method and the results achieved with it will now be described:

640 cubic meters (S.T.P.) per hour of the same cracked gas as in the comparative test in Example 1 is supplied to column 2 through line 1 in an apparatus as shown diagrammatically in FIGURE 5 at a pressure of 1.03 atm. abs. and at a temperature of C. and is washed with 165 kg. per hour of N-methylpyrrolidone supplied through line 3. The laden wash liquor flows through pipe 39 with a decrease in pressure to 0.2 atm. abs. into the degassing column 40 where it is heated to C. At the top of column.40, the expelled gas passes through line 44 in admixture with steam and N-methylpyrrolidone vapor into column 45 where the N-methylpyrrolidone vapor is washed out by washing with 5 kg. per hour of water which flows through line 48 to the top of column 45. A mixture of steam and gas in which small amounts of acetylene and also the compounds diacetylene, vinylacetylene, naphthalene, acenaphthene and acenaphthylene can be detected, is Withdrawn at the top of column 45.

A mixture of water and N-methylpyrrolidone flows through line 47 to the top of degassing column 40 in the bottom of which the vapor leaving through line 44 is produced by heating. The degassed Wash liquor is removed from the bottom of column 40 through line 41. kg. per hour thereof is passed through line 43 to the top of pre-scrubber 2, while 60 kgs. per hour flows through line 42 for distillation in still 25.

The distillated from still 25 is condensed and cooled and passed through lines 27 and 7 to the top of the absorber 6. The bulk of the wash liquor leaving degassing tower 12 is returned through lines 23 and 7 to absorber 6, while 60 kg. per hour thereof is passed through lines 49 and 3 into pre-scrubber 2.

0.475 kg. per hour of non-volatile constituents is withdrawn at 28. The wash liquor in the bottom of column 41 contains 0.79% 'by weight of dissolved polymers and in the bottom of column 12 0.07% by weight of dissolved polymers.

The remaining amounts, pressures and temperatures and also the function of the remainder of the apparatus correspond to those on the comparative section of Example 1. Here again the low content of polymers in the wash liquors when working according to this invention is clearly demonstrated.

We claim:

1. In a process for separating acetylene from gas mixtures which have been obtained by the cracking of hydrocarbons, .by washing the gas mixture with a solvent for acetylene in a preliminary washing, washing out the acetylene and at least a portion of gaseous compounds having better solubility than acetylene from the gas mixture in a main washing under pressure with a main washing liquor containing a larger amount of solvent than in the preliminary washing, passing said main washing liquor and dissolved gases to a stripping tower, expelling a. major portion of the acetylene from the main washing liquor in said stripping tower, passing said stripped main Washing liquor to a degassing tower, expelling the remainder of acetylene in said degassing tower and returning said acetylene to said stripping tower, purifying said acetylene in countercurrent contact with the main Washing liquor in said stripping tower, removing from the main washing liquor the compounds of better solubility than acetylene by means of a member selected from the group consisting of steam and solvent vapor and mixtures thereof and withdrawing an efiiuent mixture of the gaseous compounds having better solubility than acetylene and a member se lected from the group consisting of steam and solvent vapor and mixtures thereof, the improvement which comprises treating the wash liquor obtained from said preliminary washing by bringing said Wash liquor obtained from said preliminary washing into contact with said eflluent mixture selected from the group consisting of steam and solvent vapor, the compounds having better solubility than acetylene and mixtures thereof, and thereafter returning at least a portion of said treated wash liquor to said preliminary washing.

2. A process as claimed in claim 1 wherein the wash liquor from the preliminary washing is brought into contact with a countercu'rrent flow of an effluent mixture.

3. A process as claimed in claim 1 wherein the wash liquor from the preliminary washing which has been thus treated, is freed from absorbed constituents of poor volatility by distillation and is recycled together with the degassed main washing liquor from the main Washing and any remainder of degassed wash liquor from the preliminary washing to the two washings.

References Cited UNITED STATES PATENTS 2,685,941 8/1954 Kassel 55-48 2,738,859 3/1956 Bartholome et a1 55-64 5 2,796,951 6/1957 Bogart 55-65 2,870,867 1/1959 Bartholome et al. 55-65 REUBEN FRIEDMAN, Primary Examiner.

10 C. N. HART, Assistant Examiner. 

1. IN A PROCESS FOR SEPARATING ACETYLENE FROM GAS MIXTURES WHICH HAVE BEEN OBTAINED BY THE CRACKING OF HYDROCARBONS, BY WASHING THE GAS MIXTURE WITH A SOLVENT FOR ACETYLENE IN A PRELIMINARY WASHING, WASHING OUT THE ACETYLENE AND AT LEAST A PORTION OF GASEOUS COMPOUNDS HAVING BETTER SOLUBILITY THAN ACETYLENE FROM THE GAS MIXTURE IN A MAIN WASHING UNDER PRESSURE WITH A MAIN WASHING LIQUOR CONTAINING A LARGER AMOUNT OF SOLVENT THAN IN THE PRELIMINARY WASHING, PASSING SAID MAIN WASHING LIQUOR AND DISSOLVED GASES TOA STRIPPING TOWER, EXPELLING A MAJOR PORTION OF THE ACETYLENE FROM THE MAIN WASHING LIQUOR IN SAID STRIPPING TOWER, PASSING SAID STRIPPED MAIN WASHING LIQUOR TO A DEGASSING TOWER, EXPELLING THE REMAINDER OF ACETYLENE IN SAID DEGASSING TOWER AND RETURNING SAID ACETYLENE TO SAID STRIPPING TOWER, PURIFYING SAID ACETYLENE IN COUNTERCURRENT CONTACT WITH THE MAIN WAHING LIQUOR IN SAID STRIPPING TOWER, REMOVING FROM THE MAIN WASHING LIQUOR THE COMPOUNDS OF BETTE SOLUBILITY THAN ACETYLENE 